Rotary switch of the aforementioned type exists and are commonly implemented in automotive environment for instance to control the air conditioning or a Hi-Fi system.
Said switches are typically built on an electronic printed circuit board (PCB). A tubular cylindrical core is fixed on the PCB, and serves as a primary guide for a bushing that is placed over said cylindrical core. An external rotary knob, accessible to an operator, is placed over the bushing. The switch is in mechanical and electrical connection with the PCB and the rotation of the knob selects various functions. Furthermore, similarly to a key on a computer key board, a function chosen by rotation can be validated by a push on the knob, which then axially slides on the bushing toward the PCB and commutes an electrical switch. An elastic mean, such as a coil spring, biases the knob away from the pushed position, where a function has been validated, back to an extended position, when not pressed by the operator. To help the operator in the function selection, a liquid crystal display (LCD) may be fixed on the core while the knob remains open or provided with a transparent window in order to leave a visual direct access to the LCD. A back illumination of the LCD is made possible as the hollow center of the cylindrical core is a light channel for a light beam generated by a light source, typically a light-emitting diode (LED) fixed on the PCB.
In EP1555684, Kikuya et al. disclose such a rotary switch. This and other switches of the same type have been successfully implemented in diverse environments including inside many vehicles. In US2004/0154910, Hayashi discloses a rotary switch having a knob guided on a fixed cylindrical core between the lower portion of the core on which the knob slides and, the upper portion of the core where a set of balls maintained in individual radial cavities are radially pressed by springs against the knob's inner cylindrical surface. Consequently to this arrangement, each ball slides between the surfaces of the knob, of the cavity and the final turn of the spring.
Unfortunately all these switches suffer from characteristics inherent to their structure. Indeed, the sliding of the knob and of other elements result in an unpleasant friction feeling perceived by the operator. Furthermore, said friction goes against a desired accuracy of the positioning of the knob. Even though the switch may be provided with an indexing feature, the friction generates a need to manually slightly adjust the angular position of the knob. Also, after being pushed to validate a function, the friction acts against a self-return of the knob in the extended rest position. In an attempt to minimize the friction, the functional gap between the knob and the bushing has to be increased above the mandatory minimum required to accommodate the manufacturing tolerances. Consequently, under small lateral forces, the knob is subject to a very unpleasant little wobble perpendicular to the rest-pushed direction. This increases the perceived feeling of inaccuracy.
It is important to propose to the market a rotary switch having a push function that is solving aforementioned problems in having the desired wobble-free precise and accurate motion of the knob.